Fungal pathogens are a worldwide threat to human health. Candida species are the most common fungal pathogen of humans and are responsible for a wide spectrum of clinical disorders that are associated with significant morbidity and mortality. Candida is a prevalent cause of debilitating mucosal infections in the general population, and life-threatening systemic infections in hospitalized patients. Candida albicans and Candida glabrata, rank first and second in isolation frequency, respectively, and together are responsible for the majority of Candida infections. Despite its clinical significance very litte is known about the innate protective factors that provide the first line of defense against these pathogens. This is particularly true for the understudied and potentially antifungal resistant pathogen C. glabrata. Complement plays an important role in immunity to many pathogens: by direct killing through the formation of a membrane attack complex (MAC), by acting as an opsonin that facilitates microbial uptake by phagocytes, and by inducing inflammation. Fungal pathogens are protected from MAC-mediated lysis because their thick cell wall presumably blocks the formation of a lytic pore. Thus, the role of complement in antifungal innate defense has been attributed to its opsonic and inflammatory activity and it was assumed that complement does not have a direct antifungal effect. The original notion that complement activation does not directly kill Candida species has been overturned, because we have discovered that complement peptides (C3a, C4a, and C5a) have potent antifungal activity against all Candida species tested, including C. glabrata. In this R15 proposal, we will elucidate the molecular mechanisms responsible for the antifungal activity of complement peptides against clinically relevant Candida species. Based upon our preliminary data we HYPOTHESIZE that C3a interacts with Candida cell wall mannoprotein and/or -glucan. This interaction leads to peptide entry, which causes membrane permeabilization, loss of ATP (energetic potential), and mitochondrial dysfunction. Ultimately, these events induce changes in gene expression and trigger programmed cell death. In Aim 1, we will use biochemical and genetic approaches to identify the role of Candida cell wall components in modulating C3a activity. In Aim 2, we will use a candidate and non-candidate approach to identify the intracellular events and targets that are critical for C3a killing. Our long-term goal is to understand infection strategies of Candida species and the host protective factors that control the outcome of the host-fungal interaction. This AREA project will further advance our knowledge of host protective immunity against Candida infections, enrich the academic environment at the Medical University of South Carolina, and potentially reveal novel antifungal targets for improved prevention or therapy of fungal infections.